gitk: Integrate the reset progress bar in the main frame

gitk: Integrate the reset progress bar in the main frame

This makes the reset function use a progress bar in the same location
as the progress bars for reading in commits and for finding commits,
instead of a progress bar in a separate detached window.  The progress
bar for resetting is red.

This also puts "Resetting" in the status window while the reset is in
progress.  The setting of the status window is done through an
extension of the interface used for setting the watch cursor.

Signed-off-by: Paul Mackerras <paulus@samba.org>